Separation technologies using separation membranes have recently been making remarkable progress. Such separation technologies include various examples, such as separation between liquid and solid, e.g., obtaining drinking water by separating impurities, and separation of gases, e.g., oxygen-enrichment by separating nitrogen from air. Particularly, in gas separation technologies, the establishment of technologies to separate carbon dioxide selectively from a mixed gas is desired earnestly from the viewpoint of efficient recovery of fossil resources or prevention of global warming and promoted actively.
However, since conventional polymer membranes can exhibit only insufficient carbon dioxide selectivity (the membrane permeability of carbon dioxide/the membrane permeability of other gases), carbon dioxide of a desired concentration has not been recovered successfully therewith. In order to obtain a separation membrane superior in carbon dioxide selectivity, the use of a material with high affinity to carbon dioxide has been proposed; for example, there has been proposed a separation membrane in which a microporous support has been impregnated with a polyamidoamine dendrimer that in the form of a liquid material at room temperature (non-patent documents 1 and 2). Although this impregnated membrane exhibits high carbon dioxide selectivity with a value of 1,000 or more under such a condition that no pressure difference is applied to the membrane, it is insufficient for practical use because of its low membrane flux due to no pressure difference. On the other hand, it has not been used practically also under the application of pressure because the polyamidoamine dendrimer flows out from the support with time under the application of pressure to become impossible to maintain its selectivity.
As a method for solving this problem, there has been proposed a composite membrane in which a layer of a hydrophilic macromolecular material crosslinked with a crosslinking agent as a matrix impregnated with a specific amine compound has been formed on the surface of a porous support membrane (patent document 1). This composite membrane is considered to be a separation membrane that not only is high in carbon dioxide selectivity but also can withstand a certain pressure difference. Where water vapor is contained in the mixed gas to be subjected to separation, however, there are required properties contrary to each other, i.e., a moderate hydrophilicity with which affinity is exhibited between the mixed gas and a membrane surface and water resistance with which the separation membrane exhibits no structural change under a water vapor atmosphere. The aforementioned composite membrane is difficult to be used for practical use because if a mixed gas is supplied under a water vapor atmosphere, then the contained amine compound flows out from the composite membrane with time and the composite membrane cannot maintain its carbon dioxide selectivity. Since the number of examples of inclusion of water vapor in a mixed gas is expected to increase greatly due to the establishment of coal gasification power plants, there has been desired earnestly the development of a separation membrane that has moderate hydrophilicity and water resistance and can be used practically even for a mixed gas containing water vapor.